Annual dynamics of periglacial alluvial fans mapped and quantified using time series of UAV data in Svalbard

The location of alluvial fans at the intersection of the lithosphere, hydrosphere, and atmosphere makes them valuable in recording long-term environmental changes. Short-term (annual) surface morphological changes of fans are also important, as they provide a geomorphological record of high-magnitude, low-frequency processes like debris flows or avalanches, which can pose a risk to human life, infrastructure, and cultural heritage. Our study focused on the annual dynamics of fans in Spitsbergen, Svalbard. This high-Arctic location is essential because it provides insight into the dynamics of fans in an area where climate warming is progressing at one of the highest rates on Earth, which can potentially lead to an increase in the frequency and/or magnitude of geomorphological processes. We quantified the geomorphological changes in surfaces of four debris-flow-dominated alluvial fans non-affected by direct human activity. High-resolution (cm-scale) elevation data were collected using a time series of UAV surveys conducted annually between 2015 and 2019. Our research has shown that most of the area (88–99 %) of individual fans remained stable during the studied period. However, we were still able to identify significant morphological changes using UAV data when high-magnitude, low-frequency processes such as debris flow, avalanche, or ground collapse occurred. The area of the individual fan that experienced elevation changes greater than the minimum level of detection (0.10 m) varied depending on the year, ranging from 0.2 % to approximately 8 %, with significant spatial and temporal variability. The volume of changes ranged from −2200 m³ to +4000 m³ per year, with the highest recorded erosion being 5 m and the highest deposition being 2.1 m per year. The variation in geomorphological response was caused by multiple factors, including fan and catchment morphometry, surface composition, timing of specific events, presence of permafrost, and the diversity of geomorphological processes that transformed fan surfaces.